Gerald I. Evenden wrote:
>A far as the Algerian system(s) you are discussing, I finally
>guessed that you
>are referring to the transition from one grid system to another as one treks
>southward. So? What is new about that? In a little postage stamp size
>state like Massachusetts we have two grid systems: one for the mainland and
>another that covers the islands of Marthasvinyard and Nantucket. California
>has 6 grid systems with irregular boundaries determined by county borders.
>At least the Algerians and UTM have regular grid boundaries.
>>The situation in Algeria is the rule not the exception.
>>Welcome to the world of grid systems.
At least the above-mentioned are documented. In the course of
implementing the Old Egyptian grid (1907) which has four documented
(EPSG) areas called Belts, I calculated the grid values at for the
lat/lons at the corners of each "Belt" to set range checks for user
data entry and thus discovered that the main "Red Belt" which
straddles the Nile valley has negative coordinates in the south near
the border with Sudan, and the "Purple Belt" has one much further
north and west.
Ragab Hafiez of DASCO in Cairo investigated this at the Egyptian
Survey Authority at my request and discovered that the most recent
1:50000 maps have an undocumented False Northing whereby 1,000,000
was added to the published FN of 810,000 used north of the area in
question. That has led me to invent my own "Red Extended Belt" south
of 22 deg 45 min which is not documented in EPSG (but perhaps should be?).
California is in "good" (sic) company with the other side of the
Pacific. The Japan grid follows the boundaries of prefectures in
places cutting across islands and has 19 zones stretching across a
vast area, not to mention having grids with Bessel and GRS80
ellipsoid datums as well. Further entertainment is provided by the
old Austrian and the old Japan Bessel grids which have negative E-W
coordinates as well.
Can anyone suggest a solution or reference to the apparently trivial question:
Given a lat/lon value, in which zone or area does the point fall so
that the FN, FE and origins can be determined automatically by
either table lookup or a point in polygon search? The first thing
which comes to mind suggests using the Euler solution to the "point
in polygon" problem, or more subtly employing data structures like
KD, BSP or BAR trees but in most cases one doesn't have data for
boundaries in machine readable form.
Irwin Scollar